1. Field of the Invention
The invention relates generally to spin transfer torque magnetoresistive random access memory (STT-MRAM) with perpendicular magnetic anisotropy (PMA), and more particularly to PMA STT memory cells with double MgO interfaces.
2. Background of the Invention
Spin transfer torque magnetoresistive random access memory (STT-MRAM) with magnetic tunnel junction (MTJ) memory cells having perpendicular magnetic anisotropy (PMA) is a strong candidate for future non-volatile memory. The MTJ memory cell has a free ferromagnetic layer (also called the recording layer or storage layer) and a reference ferromagnetic layer (also called the pinned layer) separated by a thin insulating tunnel barrier, which is typically MgO. The free and reference layers have PMA with magnetizations oriented perpendicular to the planes of the layers. The magnetization of the reference layer is pinned, but switching current through the MTJ causes the magnetization of the free layer to switch between the two orientations, parallel (P) or antiparallel (AP) to the reference layer magnetization, corresponding to resistances RP or RAP. The tunneling magnetoresistance (TMR) of the MTJ is represented as (RAP−RP)/RP. FIG. 1A is a schematic showing the two possible states of the MTJ. FIG. 1B is a highly schematic representation of a MRAM with an array of MTJ memory cells connected to word and bit lines in a well-known cross-point architecture.
In CoFeB/MgO/CoFeB PMA MTJs, the PMA originates from the interface between the CoFeB and MgO and has been attributed to hybridization of Fe 3d and O 2p orbitals. Since this PMA originates from the interface, there is thickness limitation of the free layer because thicker layers will exhibit in-plane anisotropy. Because the free layer of a PMA MTJ with a single CoFeB/MgO interface may not provide sufficient PMA for high thermal stability, a PMA MTJ with a double CoFeB/MgO interface structure has been proposed. [“Properties of magnetic tunnel junctions with a MgO/CoFeB/Ta/CoFeB/MgO recording structure down to junction diameter of 11 nm”, H. Sato et al., Applied Physics Letters 105, 062403 (2014)]. In this structure the CoFeB free layer is sandwiched between the MgO tunnel barrier and an upper MgO capping layer, with a Ta cap formed on the MgO capping layer. However, this type of structure requires precise growth of the MgO layers while preventing the diffusion of Ta into the MgO layers during the annealing process.
What is needed is a double interface PMA MTJ memory cell with improved PMA and prevention of Ta diffusion during annealing.